1. Field of the Invention
This invention relates to the field of operational amplifiers (op amps), and particularly to rail-to-rail op amps suitable for use in a line-inversion grayscale reference generator.
2. Description of the Related Art
LCD display panels are made up of pixels, with each pixel's transparency varying with the voltage applied across it. The “source driver” circuit which provides the voltages applied to the pixels is typically a simple digital-to-analog converter (DAC). The DAC consists of a series string of resistors with a voltage connected across it such that unique voltages are produced at each resistor—resistor junction, followed by a multiplexer which passes on one of the produced voltages in response to a digital code. As the digital code to the mux is increased from zero-scale to full-scale, the voltage applied across a pixel increases linearly. Unfortunately, the relationship between pixel transparency and applied voltage is non-linear; as such, applying a drive voltage which is ¼ of full scale may not result in a ¼ brightness level from an addressed pixel.
One approach to correcting the non-linearity involves “bending” the DAC at certain points along the resistor string, using a grayscale reference generator. Several correction voltages are applied to selected resistor—resistor junctions, such that the DAC's transfer characteristic is made linear between pairs of correction points (though the DAC's overall transfer function may now be non-linear). When properly arranged, linearly incrementing the digital code to the DAC causes the transparency of a driven pixel to vary linearly.
A basic grayscale reference generator 10 and source driver 12 are shown in FIG. 1; a grayscale reference generator in combination with a source driver are referred to herein as an “LCD display driver”. A pixel 14 is addressed by toggling a “gate” line (G) for the row in which the pixel resides, which closes a switch 16 to connect the pixel to a particular column line, which is driven with a source driver as described above. A desired drive voltage is applied to the pixel's column line, and is stored on a storage capacitor 18.
Another problem associated with LCD displays is “ghosting”. Ghosting is avoided by maintaining a near-zero average DC voltage across the pixels. This is accomplished by periodically alternating the polarity of the voltage applied across each pixel so as to maintain a constant absolute voltage across it. For example, in FIG. 1, ghosting is avoiding by connecting the “second” terminal (19a) of each pixel 14 to a voltage VCOM, which is alternately switched between first and second voltages—typically a positive supply voltage and ground, but any two voltages could be used. Simultaneously, the polarities of the drive voltages applied to the pixel's “first” terminal (19b) are alternated in synchronization with the switching of the second terminal so as to maintain a constant absolute voltage across the pixel. A generator of this type is referred to herein as a “line-inversion” LCD grayscale reference generator.
One way of alternating the polarities of the drive voltages is shown in FIG. 1. Voltage sources (not shown) generate a set of correction voltages (VA, VB, VC, VD, VE) for use when the pixel is being driven with a first polarity, and a set of correction voltages (VA′, VB′, VC′, VD′, VE′) for use when the pixel is being driven with the opposite polarity. An analog multiplexer 20 receives the two sets of corrections voltages, and switches one set or the other to the resistor—resistor junctions of source driver 12 in synchronization with the switching of pixel terminal 19a. A controller 22 operates analog mux 20, source driver 12, and the switching of the pixel terminal.
Amplifiers A1–A5 are typically interposed between respective outputs of analog multiplexer 20 and source driver 12. Most of the amplifiers, particularly the “outer” amplifiers (A1, A2, A4 and A5 in this example), need to be able to swing close to their supply rails (V+ and ground in this example); a graph illustrating the input voltages that amplifiers A1–A5 might need to accommodate is shown in FIG. 1b. This requires that these outer amps be rail-to-rail amplifiers.
A typical rail-to-rail amplifier as might be used in grayscale reference generator 10 is shown in FIG. 1c. A PMOS differential input pair 24, 26 receives tail current from a current source 28, and an NMOS differential input pair 30, 32 receives tail current from a current source 34. The gates of PMOS FET 24 and NMOS FET 30 share a common input terminal (+), as do the gates of PMOS FET 26 and NMOS FET 32 (−). The pairs generate respective differential output currents that vary with the differential input voltage applied across the input terminals; an output stage 36 generates an output voltage (OUT) which varies with the differential output currents.
There are several drawbacks to the use of a rail-to-rail amplifier as shown in FIG. 1c. One disadvantage is that for some values of common-mode input voltage (Vcm), both input pairs will be operational, while for other values of Vcm only one pair is operational. Having both pairs active consumes what may be an excessive amount of supply current.
Another disadvantage concerns the amplifier's frequency compensation scheme. Such a scheme is designed to limit the maximum amplifier bandwidth to avoid oscillation. Bandwidth varies with the transconductance of the input stage, and is greatest when both pairs are active. As such, the amplifier's compensation scheme is generally optimized for this condition. This means that the compensation becomes sub-optimal—and the amplifier's bandwidth unnecessarily limited—when only one pair is active.